Optical systems are typically compiled of several optical elements. In optical elements and in systems compiled thereof, there always occur imaging errors. In optical planning, the aim is always the best possible result from the point of view of the end application. A general aim is that an object point under observation could be imaged as an image point. For instance in a camera and other optical systems designed for image reproduction, it is necessary to take into account, in addition to the features of a possible display device, also the resolution of the human eye, when the required image accuracy and pixel size are being iterated. In general, resolution means the capacity of a system to distinguish details. Resolution is typically expressed as the shortest angular distance between two points that are only just distinguished as two separate points. The resolution of a healthy human eye in favourable conditions is not more than one arc minute.
Typically optical imaging errors are corrected or compensated by suitable combination of various lenses. A lens or a lens combination with a positive focal length is generally called a positive lens. Positive lenses that collect rays are for instance a convex-convex or a piano-convex lens. Respectively lenses that have a negative focal length, i.e. dispersing, concave lenses or lens combinations, are generally called negative lenses. When correcting imaging errors, there is chosen a suitable glass quality, lens thickness and shape, the location of the apertures and the lens coating. Typically when planning an optical system, there is searched an appropriate balance between the different imaging errors, in order to achieve a sufficiently good result in each case.
Often the image quality is better, the larger the number of lenses that is used for producing the image. For instance from the point of image reproduction, an ideal optical system could comprise 20 lenses, because when correctly combined, different glass and lens types mutually compensate their imaging errors. However, if the object is to realize light-weight optics in a small space, the number of lenses must be kept remarkably smaller, which means that the image quality requirements must be cut down. It can be roughly maintained that the performance of an optical system is always better, the larger the number of lenses that it is built of. However, when planning the system, it must also be taken into account that about 5% of light is reflected back from each lens. Apart from the fact that in multi-lens systems luminosity is decreased, systems containing several elements are heavy and expensive. Even if a good image quality is achieved, the size, weight and expenses restrict the applicability and usage of this kind of optics.
A known device where optics are generally used is a camera. A prior art optical solution designed for a small camera is disclosed in the publication U.S. Pat. No. 6,342,975. In the lens system, the first and second lens groups are positive, and the third lens group Is negative. The first lens group includes a negative and a positive lens. The second lens group includes an adjustable aperture and a negative and a positive lens. The third lens group includes a positive and a negative lens. When the optics are zoomed from a wide-angle close-up view to a telescopic telephoto view, each of the lens groups moves towards the target end, i.e. towards the first lens group. In addition, the distance between the first and second lens group grows, and the distance between the second and third lens group is shortened. This arrangement, however, contains a large number of single optical elements, which makes the optical arrangement expensive. Moreover, when the number of lenses increases, the optical system requires more space, and it becomes heavier. The mutual positioning of the optical elements and the realization of their mechanical mobility as well as their control require precision.
Another simple lens system suited in a small camera is introduced in the publication U.S. Pat. No. 6,040,949. The arrangement aims in that both the camera and its optical system can be realized in a small size, as small as possible. The optical system includes a lens system, and the three lens groups thereof are illustrated in FIG. 1 representing the prior art. The first lens group 101 in FIG. 1 constitutes a plano-concave lens, and it has a negative refractive power. The second lens group 103 in FIG. 1 constitutes a convex-convex lens, and it has a positive refractive power. The third lens group 104 in FIG. 1 constitutes combined convex-convex and plano-concave lenses, and the refractive power of the third lens group 104 is positive. The target under observation that should be pictured, illustrated in FIG. 1, is located on the left side 105 of the lens group 101, and an image of the target is formed on the right side 106 of the lens system. The optical axis 107 runs through the center point of the lenses. Between the first lens group 101 and the second lens group 103, there is an adjustable aperture 102. When focusing the image, i.e. when zooming a certain part of the view, the locations of the second lens group 103 and of the third lens group 104 are changed along the optical axis. The described arrangements advantageously include only 4 single lenses.
In the above cited publications, the lens groups are moved along the optical axis in order to adjust the focal length of the system. The reciprocal moving of lenses is a very typical way to adjust the optical properties of the system, such as focal length, magnification, focusing or angular field. Another known way, particularly used in cameras, to adjust the focal length or the refractive index is to replace the lens by a different type of lens. This can be realized for instance so that in front of the camera lens, there is installed a disc containing different types of lenses that can be changed in order to picture the chosen target as accurately as possible. The mechanical moving of the lenses or of the disc always requires space, which is minimal in small devices. In addition, the mechanical adjustments must be carried out accurately both in the installation step and when the device is being used, in order to arrange the optical components precisely at the right position in relation to the optical axis and to each other.
A typical prior art optical system suited in small cameras is a system with only one focal length, in which case the focal length is not adjusted mechanically at all. In this arrangement, the distance from the target to the camera falls within a fairly limited range if a good image quality should be maintained. This means that the image only has a high quality within a given average range, but a sharp image cannot be made of targets located at a close or distant range. Consequently the possibility to adjust the focal length is a desired feature in optical applications and in devices utilizing them.